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potasse sur l’évolution minéralogique des argiles du sol
Eleanor Bakker
To cite this version:
Eleanor Bakker. L’impact de la croissance des plantes et l’absorption du potasse sur l’évolution minéralogique des argiles du sol. Earth Sciences. Université Grenoble Alpes, 2018. English. �NNT : 2018GREAU012�. �tel-01835126�
THÈSE
Pourobtenirlegradede
DOCTEUR DE L’UNIVERSITÉ GRENOBLE ALPES
Spécialité: SciencesdelaTerre,del’Universetdel’Environnement
Arrêtéministériel:25mai2016
Présentéepar
Eleanor Bakker
ThèsedirigéeparBrunoLanson etcodirigéeparFabienHubert
préparéeauseindel’InstitutdesSciencesdelaTerre etdel’écoledoctoraleTerreUniversEnvironnement
The impact of plant growth and potassium uptake on clay minerals in soil
Thèsesoutenuepubliquementle6avril2018, devantlejurycomposéde:
PhilippeHinsinger
DirecteurdeRecherches,INRAUMREco&Sols,Montpellier,Rapporteur
StephenHillier
ProfessorattheJamesHuttonInstitute,Aberdeen,Royaume-Uni,Rapporteur
LaurentCharlet
ProfesseurauISTerre,UniversitéGrenobleAlpes,Grenoble, Président
PierreBarre
ChargédeRechercheauEcoleNormaleSuperior,Paris,Examinateur
FabienHubert
Maître deConférence auIC2MP, UniversitédePoitiers,Grenoble,Co-Directeurde thèse
BrunoLanson
R´ esum´ e
Le potassium est un nutriment essentiel `a la croissance et au d´eveloppement des plantes.
Les min´eraux argileux dans les sols repr´esentent un important r´eservoir de K disponible pour les plantes. L’extraction de potassium fix´e `a partir de l’espace interfoliaire des min´eraux micac´es 2:1 peut entraˆıner une augmentation de la distance feuillet `a feuillet qui peut ˆetre mesur´ee par diffraction des rayons X . Des ´echantillons de l’exp´erience Morrow Plots continue avec du ma¨ıs ou du ma¨ıs-avoine-foin, provenant de sous-parcelles fertilis´ees et non fertilis´ees pour les ann´ees 1904, 1957, 1980, 1997 et 2013-2014, ont ´et´e soumis au fractionnement granulom´etrique s´equentiel pour obtenir la fraction limoneuse (50-2 µm) et les sous-fractions argileuses (2-0,2, 0,2-0,05 et <0,05 µm). Les r´esultats granulom´etriques montrent une h´et´erog´en´eit´e significative malgr´e la petite taille de la MP, et un gain de sous-fraction< 0,05 µm avec le temps. La mod´elisation des diagrammes de diffraction des rayons X a ´et´e effectu´ee pour obtenir une identification concluante de l’assemblage de min´eraux argileux et ´evaluer l’impact de 110 ans d’agriculture continue et de diff´erentes pratiques agronomiques de 1904 jusqu’`a 2014. Un assemblage complexe de min´eraux argileux a ´et´e identifi´e avec jusqu’`a onze contributions diff´erentes n´ecessaires pour reproduire les donn´ees exp´erimentales de sous-fractions de moins de 2µm, y compris jusqu’`a six couches mixtes d’illite-smectite-chlorite. L’analyse de phase quantitative pour toutes les sous-parcelles et toutes les ann´ees, a montr´e que l’assemblage min´eral du Morrow Plots est similaire entre les diff´erentes sous-parcelles, quel que soit le traitement agronomique. Aucune preuve significative d’alt´eration ou de transformation des phases min´erales argileuses n’a ´et´e observ´ee au fil du temps. La nature diocta´edrique des min´eraux argileux de la MP d´esavantage l’extraction du potassium et donc la dissolution est le m´ecanisme privil´egi´e pour l’absorption du potassium et la perte de sous-fractions 2-0,2 et 0,2-0,05 µm est attribu´ee `a ce processus.
Abstract
Potassium is an essential nutrient for plant growth and development. Clay minerals in soils represent an important reservoir of plant-available K. Extraction of fixed potassium from the interlayer space of micaceous 2:1 minerals can lead to an increase in the layer- to-layer distance which can be measured by X-ray diffraction. Samples from the Morrow Plots continuous corn and corn-oats-hay experiment, from fertilised and non-fertilised subplots for the years 1904, 1957, 1980, 1997 and 2013-2014 were subjected to sequential size-fractionation to obtain the silt fraction (50-2 um) and clay-sized subfractions (2-0.2, 0.2-0,05 and <0.05 um). Granulometric results show siginificant heterogeneity despite the small size of the MP, and a gain in <0.05 um subfraction with time. Full-profile fitting of X-ray diffraction patterns was performed to obtain conclusive identification of the clay mineral assemblage and to assess the impact of 110-years of continuous agriculture and different agronomic practices from 1904 to 2014. A complex clay mineral assemblage was identified with up to eleven different contributions necessary to reproduce the experimental data of <2 um subfractions, including up to six illite-smectite-chlorite mixed-layers. Quantitative phase analysis for all subplots and years showed that the mineral assemblage of the Morrow Plots is similar between different subplots, regardless of agronomic treatment. No significant evidence of alteration or transformation of clay mineral phases was observed over time. The dioctahedral nature of the clay minerals of the MP disfavours potassium extraction and thus dissolution of the entire crystal structure appears to be the favoured mechanism of nutrient extraction. The loss of 2-0.2 and 0.2-0.05 um subfractions is attributed to this process.
Acknowledgements
First of all I express my gratitude to my supervisors, Bruno Lanson and Fabien Hubert, firstly for choosing to accept me for the PhD position, and secondly, for the invaluable advice and guidance through my PhD and the mountains of work involved. I really appreciate the assistance you proivded and the confidence you had in me to complete the work.
Thanks also to my rapporteurs, Stephen Hillier and Philippe Hinsinger, for accepting the task of reading my manucript and providing insightful comments and ideas. I also appreciate the input of my examinateurs Michelle Wander, Pierre Barr´e and Laurent Charlet for taking the time to assess my work and for the questions and feedback that you have given me.
I would also like to thank Nathaniel Findling for all his help with sample preparation, his patience dealing with my basic French at the start of my PhD, for running so many XRD analyses for me, and most importantly for his care repairing the centrifuge when it had tomb´e en panne. I would also like to extend my thanks to Martine Lanson for performing CEC and elemental analyses for me, and all her other assistance in the lab.
And further thanks to all the other interesting and wonderful people in ISTerre who were always welcoming, whether over a cup of coffee or celebrating with an inflatable unicorn.
sary distractions in the many mo- ments we spent talking-laughing- drinking-dancing-relaxing-picnicking-playing. In no particular order, Natalie, Laura, Jess, Michelle, Nikos, Wilfrid, Paul, Eric, Maor, Jakes, Tobias, Thu, Benoit, Irene, Frans, Anca, Favio, Thu, Liva, Zane, Christian, Randy, Moritz, Catarina, Cyril, Igor, Marcelo, Margarita, Felix, Matt, Will, Linda, Miguel, and probably many others who
I have forgotten (sorry!). Special thanks to Maor for market coffee, Rummikub, and for tolerating the worst of my stressed moods; to Laura as a wonderful and easygoing
flatmate; to Natalie for general awesomeness and Princess provider; and to Irene for all the dirty jokes and quotes. And last but not least a big thanks to my fam-
ily for all the conversations and support during my #PhDstruggle, it would have not been possible without you. Big thanks to Arwen for knowing
the right things to say to kick me out of my ruts, to Dylan, Maria, and Willem (and cats!) for giving me a relaxing hideaway
in Berlin and not complaining (much) that I spent all my time gaming, and to Benny and Kirstie for
their massive confidence in me and the rational advice I needed.
Love you guys!
♥
Contents
1 Introduction 15
1.1 Potassium . . . 15
1.1.1 General information . . . 15
1.1.2 Uses of potassium . . . 16
1.1.3 Occurrence and extraction . . . 18
1.2 Potassium as an essential plant nutrient . . . 19
1.2.1 Essentiality and use . . . 19
1.2.2 Potassium deficiency and excess . . . 21
1.2.3 Bioavailability and uptake . . . 22
1.2.4 Potassium cycling . . . 23
1.3 Soils and pedogenesis . . . 25
1.3.1 Soil . . . 25
1.3.2 Soil forming factors . . . 26
1.3.3 Climate and topography . . . 26
1.3.4 Parent material . . . 27
1.3.5 Biota . . . 28
1.3.6 Effect of time . . . 29
1.3.7 Phases of soil formation . . . 29
1.4 Soil structure and definitions . . . 30
1.4.1 Particle aggregation . . . 30
1.4.2 Definition of soil horizons . . . 31
1.4.3 Pedon . . . 32
1.5 Soil components . . . 32
1.5.1 Humus and humification . . . 32
1.5.2 Inorganic soil constituents . . . 34
1.5.3 Structural building blocks . . . 34
1.6 Minerals of interest . . . 35
1.6.1 Feldspars . . . 35
1.6.2 Phyllosilicates . . . 36
1.6.3 Micas and 2:1 clay minerals . . . 37
1.6.4 Layer stacking and disorder . . . 39
1.7 Characteristics of 2:1 minerals . . . 39
1.7.1 Layer charge . . . 39
1.7.2 Cation exchange capacity . . . 40
1.7.3 Layer-to-layer distance . . . 40
1.8 Setting of the study . . . 41
1.8.1 Mineralogical transformations by plants . . . 41
1.8.2 Long-term fertility experiments . . . 43
1.8.3 Position of the study . . . 44
2 Materials and methods 47 2.1 The Morrow Plots experiment . . . 47
2.1.1 Description and location . . . 47
2.2 Experimental procedures . . . 51
2.2.1 Preparation of samples for XRD . . . 51
2.2.2 Chemical analyses . . . 52
2.3 Analysis by X-ray diffraction . . . 52
2.3.1 Principles of XRD . . . 52
2.3.2 XRD of clay minerals . . . 54
2.3.3 Quantitative analysis of samples . . . 54
2.3.4 Initial modelling approach . . . 55
2.3.5 Inclusion of K-150 samples . . . 57
2.3.6 Simulation of XRD patterns . . . 58
I Identification of the mineral assemblage and pedogenetic influences of the Morrow Plots 61
3 Pedogenetic influences 63 3.1 Introduction . . . 633.2 Materials and methods . . . 65
3.3 Results . . . 67
Contents
3.3.1 Particle size distribution . . . 67
3.3.2 Chemical Analyses . . . 67
3.3.3 Qualitative interpretation of XRD . . . 68
3.3.4 XRD profile simulation . . . 71
3.4 Discussion . . . 75
3.4.1 Validity of the proposed structural model . . . 75
3.4.2 Comparison with previous assessment of the mineralogy of the Morrow Plots . . . 77
3.4.3 Evolution of clay mineralogy over time . . . 77
3.4.4 Cultivation effects on soil composition . . . 79
3.5 Conclusion . . . 79
II Mineralogical impacts of different agronomic practices in subplots of the Morrow Plots 81
4 Mineralogical differences 83 4.1 Introduction . . . 834.2 Materials and methods . . . 85
4.3 Results . . . 86
4.3.1 Particle size analysis . . . 86
4.3.2 Chemical analyses . . . 88
4.3.3 XRD qualitative analysis and XRD profile simulation . . . 88
4.4 Discussion . . . 91
4.4.1 Variability of results . . . 91
4.4.2 Clay mineral evolution . . . 93
4.4.3 Mechanism of transformation . . . 95
4.5 Conclusions . . . 96
5 Overall conclusions and perspectives 99 5.1 General Discussion . . . 99
5.1.1 Mineral assemblage and evolution of the Morrow Plots . . . 99
5.1.2 Validity of the proposed structural model . . . 101
5.2 Perspectives . . . 101
5.2.1 Improved constraints . . . 101
5.2.2 Broadening of scope . . . 102
5.2.3 Inclusion of additional experiments . . . 102
A Supplementary information 121
A.1 RU Rietveld . . . 122
A.2 RU phase analysis . . . 123
A.3 Particle size distribution . . . 124
A.4 Phase proportions . . . 125
A.5 Fitted XRD profiles . . . 129
A.5.1 1904 . . . 129
A.5.2 1957 . . . 131
A.5.3 1980 . . . 135
A.5.4 1997 . . . 139
A.5.5 2013 . . . 143
A.5.6 2014 . . . 147
A.6 Structural parameters . . . 151
List of Figures
1.1 Potassium in oil . . . 15
1.2 Evaporite formation . . . 17
1.3 World potash deposits . . . 19
1.4 Soy K-deficiency . . . 21
1.5 Soil K-cycle schematic . . . 24
1.6 Factors of soil formation . . . 26
1.7 Rainfall variation with latitude . . . 27
1.8 Soil horizons . . . 31
1.9 General soil map of Illinois . . . 33
1.10 Basic silicate mineral structural components . . . 36
2.1 The Morrow Plots experimental fields . . . 47
2.2 Layout of the Morrow Plots . . . 48
2.3 Conditions for diffraction . . . 53
2.4 Initial model fit in Ca-AD state . . . 56
2.5 Initial model fit in Ca-EG state . . . 57
2.6 K-150 intensity shift . . . 58
2.7 Fit comparison . . . 59
3.1 Particle size distribution of RU subplots . . . 67
3.2 Ca-AD and Ca-EG experimental XRD for RU subplots . . . 68
3.3 Ca-AD, K-150 and K-350 experimental XRD for RU subplots . . . 69
3.4 Time evolution of experimental XRD . . . 70
3.6 Layer-type compostion RU subplots . . . 73
3.7 Fit sensitivity . . . 76
4.1 Granulometry 1904-2014 all subplots . . . 86
4.2 Rietveld RU 1904 and 2014 . . . 89
4.3 Sybilla fits for 2014 CF and RU . . . 90
4.4 <2µm layer composition and evolution . . . 92
A.1 Rietveld fitting 1904-2014 RU subplots . . . 122
A.2 Quantitative phase results RU subplots . . . 123
A.3 1904 CU . . . 129
A.4 1904 RU . . . 130
A.5 1957 CF . . . 131
A.6 1957 CU . . . 132
A.7 1957 RF . . . 133
A.8 1957 RU . . . 134
A.9 1980 CF . . . 135
A.10 1980 CU . . . 136
A.11 1980 RF . . . 137
A.12 1980 RU . . . 138
A.13 1997 CF . . . 139
A.14 1997 CU . . . 140
A.15 1997 RF . . . 141
A.16 1997 RU . . . 142
A.17 2013 CF . . . 143
A.18 2013 CU . . . 144
A.19 2013 RF . . . 145
List of Figures
A.20 2013 RU . . . 146
A.21 2014 CF . . . 147
A.22 2014 CU . . . 148
A.23 2014 RF . . . 149
A.24 2014 RU . . . 150
List of Tables
1.1 Dry plant shoot matter . . . 20
1.2 Elemental abundance in the Earth’s crust . . . 35
1.3 Clay families and structural formlae . . . 38
2.1 Properties of the Morrow Plots soils . . . 50
3.1 Chemical analyses of RU subplots . . . 66
3.2 Structural parameters of the model . . . 72
3.3 Quantitative phase results for <2 µm subfractions . . . 74
4.1 Chemical analyses CF and RU subplots . . . 87
A.1 Particle size distribution . . . 124
A.2 XRD quantitative phase results 50-2 µm fractions . . . 125
A.3 XRD quantitative phase results 2-0.2 µm subfractions . . . 126
A.4 XRD quantitative phase results 0.2-0.05 µm subfractions . . . 127
A.5 XRD quantitative phase results <0.05 µm . . . 128
Introduction
Figure 1.1: Potassium balls stored in oil to prevent reaction with water vapour in air.
1.1 Potassium
General information
Potassium (K) is a alkali metal with the atomic number of 19 and a molar mass of 39.0983 gmol−1. It is one of the seven most abundant elements in the Earth’s crust at about 2.6%, and the 20th most abundant element in the universe (Halka and Nordstrom, 2010).
It is exceedingly reactive, so is never found as a pure metal, and must be stored in oil to prevent it from reacting violently with water vapour in air. Under normal conditions potassium has a single oxidation state, losing its outer electron to form the K+ cation, which has an ionic radius of 1.38-1.69 ˚A depending on the degree of covalent-ionic bonding (Aylward and Findlay, 2002). It is commonly found in the form of salts such as potassium chloride (KCl), potassium nitrate (KNO3) and potassium carbonate (K2CO3), which are
typically ionic in nature and relatively soluble in water. It is also found in feldspars and micas. There are three natural isotopes of potassium, stable39K and41K, and radioactive
40K which is used as an dating method for crustal rocks, as it decays into the stable
40Ar at a known rate (Aylward and Findlay, 2002; Halka and Nordstrom, 2010). While potassium was recognised as a unique chemical element as early as 1702, the pure metal was not isolated until 1807 by Humphrey Davy who extracted with the new method of electrolysis (at the same time extracting sodium) (Parsons and Dixon, 2013). He gave it the name potassium deriving from the common method of obtaining potassium salts in by burning wood in pots giving ash (pot-ash). Its chemical symbol, K, is taken from the German kali orkalium, which is in itself a derivation of the Arabic word for ’plant-ash’
(which also gave the word alkali) (Mengel et al., 2001).
Potassium is an essential element for all forms of living organisms, and is one of the nine most common elements in the human body (by mass) with a suggested daily intake in the range of 3500-4700 mg per day necessary to maintain potassium levels (Halka and Nordstrom, 2010). Potassium plays a vital role in a number of functions in homo sapiens, including (and not limited to) propagation of electrical pulses in nerve and cardiac tissue, protein synthesis, and maintenance of cell-membrane potentials and of fluid/electrolyte balances (Halka and Nordstrom, 2010). A diet rich in fruit and vegetables is normally enough to maintain potassium levels, and use of mineral supplements can endanger health as a high concentration of potassium affects the regulation of cell water content and can have disastrous effects on nerve signalling to vital organs (Parsons and Dixon, 2013).
Potassium is also an important plant nutrient and performs various vital functions.
Uses of potassium
Generally, pure potassium is seldom used due to its reactivity, and sodium compounds are used as a replacement where possible. Despite this, potassium compounds find a myriad of uses. Potassium cyanide (KCN) is famous for its historical role as a poison in various suicides and murders. More recently, potassium cyanide is used in large-scale gold mining operations, and is more often associated with the environmental concerns accompanying spills such as that of Baia Mare in 2000 (Batha, 2000). Potassium hydrox- ide, KOH, is a caustic agent used for soap production, and the manufacture of various drugs, alkaline batteries and for the unblocking of drains (Halka and Nordstrom, 2010).
Potassium iodide, KI, is often added to table salt as a source of iodine, and also in
1.1 Potassium disinfectants. Some potassium salts are brightly coloured, such as potassium manganate (KMnO4, purple) which is used as an antibacterial agent, chromate (K2CrO4) provides yellow colours for textiles and in ink, and yet another, dichromate (K2Cr2O7, orange) is used to preserve wood, develop blueprints, and colour glass (Parsons and Dixon, 2013).
Potassium carbonate (K2CO3) is used in the glassmaking process, in brewing beer, and in fire extinguishers. While not an extensive list, various other potassium salts find their uses in gunpowder and explosives (KNO3), fireworks and sparklers (KClO4), and as a sedative (KBr) (Halka and Nordstrom, 2010).
However by far the most important use of potassium salts, in both production mass and significance, is the use of potassium in fertilisers. Potash, blood and guano have all been applied historically to soils to ensure adequate nutrient supply to plants, even before the unique role of potassium was recognised in 1860 (Marschner, 1995). Initially potassium was supplied through organic sources of manure and potash produced by the burning of wood, however following the discovery of large evaporite deposits in at Stassfurt in Germany in 1856 mineral fertilisation became the norm (Barker and Pilbeam, 2015). Worldwide, 40.7 million metric tonnes (Mt) were produced in 2015, and of this 90-95% went into the production of agricultural fertilisers (Jasinski, 2017a). Projections by the United Nations Food and Agriculture Organisation (UNFAO) expect increases in production of 43 Mt per year by 2020, due to increased demand for fertilisers in Asia and South America, and demand related to crops for ethanol-based fuel production (UNFAO, 2017).
Occurrence and extraction
Potash bearing deposits occur in the form of evaporite, which are created by the evapora- tion of ancient oceanic basins or lakes (Kesler, 1994). They primarily contain a mixture of sylvite (KCl) and halite (NaCl), and other potassium, sodium, magnesium and bromine minerals known as sylvinite (Foth, 1991). Worldwide deposits of potash are estimated at approximately 250 billion tonnes, with the largest deposits found in the Northern Hemisphere. Three countries, Canada, Russia and Belarus, accounted for 60% of world production and 85% of recoverable reserves in 2016 (Jasinski, 2017b). As these are soluble deposits, mining takes place underground, often at depths of 1,000 m or greater, and deposits must be protected from groundwater intrusions. For example, shafts are commonly lined with concrete and steel shields to prevent the influx of water. As sylvinite
Figure 1.2: Schematic of evaporite formation from basin evaporation, from Kesler (1994).
is relatively soft, stability of mine shafts can also be an issue and closing of openings can occur within months. Thus, many mines do not recover all the mineral deposit but rather leave 55-65% behind to prevent collapse (Kesler, 1994).
In addition to underground mining, Israel and Jordan recover potassium from the Dead Sea, where the high salt concentration (approximately 8 times that of the Atlantic) makes such extraction commercially viable (Al-Harahsheh and Al-Itawi, 2005). Solution mining is a method which has replaced underground mining for deeper or more inacces- sible reserves. This is performed by injection of freshwater into boreholes to dissolve the sylvinite, and the resulting solution is then brought to the surface, cooled and KCl precipitates out. Nevertheless, 94% of worldwide potash production involves exploitation of solid ores (Nielsson, 1986).
Processing of solid potash ores is performed to obtain fertiliser-grade levels of purity, for example in the United States products for agricultural applications must contain 60%
K2O (min) which is equivalent to 95% KCl (Kesler, 1994). The method of processing depends on physical and chemical characteristics. The typical process involves crushing and grinding followed by froth flotation, while differential precipitation is performed in cases where the ore is finely grained (Nielsson, 1986). Halite, NaCl, tends to be the largest by-product of refining processes. Potassium sulfate (K2SO4) and potassium nitrate (KNO3) may also be produced for fertiliser needs. There is generally no easy
1.2 Potassium as an essential plant nutrient replacement for potash as a mineral fertiliser, as manure sources have low potassium values and need to be produced close to the location of use (Jasinski, 2017a). However current extractable reserves are sufficient to meet requirements for several decades and it is likely further underground deposits remain undiscovered or unexplored (Jasinski, 2017b).
Figure 1.3: Map of worldwide distribution of proven potash reserves (Uralkali, 2017).
1.2 Potassium as an essential plant nutrient
Essentiality and use
The growth and yield of plants is directly related to the abundance of various factors, such as light, CO2, moisture and a range of mineral nutrients. Mineral nutrients are required for specific enzymatic or metabolic roles, and can be classified as eithermacronutrients ormicronutrients, depending on the plant requirements. There are six macronutrients, nitrogen (N), phosphorus (P), potassium (K), sulphur (S), calcium (Ca) and magnesium (Mg), and a number of micronutrients shown in Table 1.1 (Marschner, 1995). Potassium is the second most important of these nutrients (by weight) after nitrogen, with amounts for optimal growth being between 2-5% of the plant dry weight (Marschner, 1995). There is also an interaction between different mineral nutrients, with nitrogen uptake increasing with potassium availability, and as a result, potassium is frequently applied in mineral fertilisers, along with lime (L, CaCO3 or other compounds to decrease pH), nitrogen and phosphorus (L-NPK fertilisers) (Foth, 1991; Marschner, 1995).
Table 1.1: Average concentrations of essential plant nutrients in dry plant shoot matter, adapted from Marschner (1995).
Element Symbol
µmol g−1
dry wt %
Macronutrients
Nitrogen N 1000 1.5
Potassium potassium 250 1
Calcium Ca 125 0.5
Magnesium Mg 80 0.2
Phosphorus P 60 0.2
Sulphur S 30 0.1
Micronutrients
Chlorine Cl 3 -
Boron B 2.0 -
Iron Fe 2.0 -
Manganese Mn 1.0 -
Zinc Zn 0.30 -
Copper Cu 0.10 -
Nickel Ni 0.001 -
Molybdenum Mo 0.001 -
The essentiality of potassium has been proved via numerous laboratory experiments involving limited supply of potassium. Potassium is primarily present as ionic K+, with a very small percentage incorporated into organic tissue, and is the most abundant cation in plant cytosol. It is involved in a large number of plant functions, including (but not limited to) (Barker and Pilbeam, 2015; Marschner, 1995):
• enzyme activation
• osmoregulation via – stomatal control – cell extension
– phloem transport of sugars and starch
• protein synthesis
• photosynthesis and respiration
• ion absorption and transport (cation-anion balance)
• disease resistance
1.2 Potassium as an essential plant nutrient The role of potassium in photosynthesis directly correlates to increasing yields, as photosynthesis involves the transformation of light-energy into chemical energy and thus growth of plant tissues (Marschner, 1995). The role of potassium in osmoregulation plays a vital part in the drought and frost resistance of crops (Mengel et al., 2001). In legumes with an abundant supply of potassium (rather than deficiency), supply of potassium cor- responds to improved transport of starch from leaf to other plant organs (Marschner, 1995).
Potassium deficiency and excess
Potassium deficiency is not easily recognisable, as the first sign is a retardation in the growth of the plant, such as poorly developed root systems or weak stalks, which con- tribute to lodging (non-vertical plant growth) (Mengel et al., 2001). A portion of this stunted growth is due to the role of potassium in cell turgor and extension, where an inverse relationship exists between potassium supply and cell-size of leaves, stems and storage tissues (potato tubers, carrots, etc.) (Marschner, 1995). Potassium deficient plants produce poor fruiting bodies, and have low frost, drought and disease resistance.
As deficiency advances, further symptoms appear first in older leaves, as a yellowing (or scorching) of the leaf edges resulting from the transport of mobile potassium from older to younger plant tissues (Mengel et al., 2001).
Potassium deficiency is also linked to a number of non-visible chemical changes. One effect is poor water regulation and thus a higher water demand (Mengel et al., 2001).
Additionally, soluble nitrogen compounds accumulate in the tissues of potassium deficient plants - perhaps related to the inhibition of certain protein synthetic pathways, or an effect on the activation of the nitrate reductase enzyme (Koch and Mengel, 1974). Accumulation of soluble carbohydrates and a decrease of starch content occurs, related to the reduced efficiency of the starch synthase in potassium deficient conditions (Marschner, 1995). A diamine, putrescine, also accumulates up to a factor of 80-100 times, as its synthesis is stimulated by the low pH arising under potassium deficient conditions. A divalent cation, putrescine compensates to a certain degree the K+-deficit, and additional external supply was shown to prevent visual signs of potassium deficiency (Tachimoto et al., 1992).
Occurrences of potassium excess are rare with no specific symptoms other than possi- ble depression of plant growth and yield. This is largely due to the well-regulated uptake of potassium by plants. Effects of oversupply of potassium tend to relate to a situation
Figure 1.4: Comparison of potassium deficient soy (Glycine max) leaves showing characterisitic yellowing of edges (left), next to soy receiving adequate potassium nutrition (right) (IPI, 2010).
called ‘luxury uptake0 and suppression of the uptake of other cations such as Na+, Mg2+
and Ca2+ as potassium competes strongly with these cations (ion-antagonism) and high quantities of Ca, Mg and Na are taken up at higher rates when potassium concentrations are low (Fageria, 1983; Foth, 1991).
Bioavailability and uptake
Plant uptake of mineral nutrients occurs through root uptake of ions in the soil solution.
Root activity influences a volume in proximity to the roots which is defined as the rhizosphere (Hinsinger, 1998). The volume of the rhizosphere can vary widely depending on the mobility of the element considered, and may even vary widely for a single element, dependent on bulk soil properties and plant species and age, but is generally considered as several mm of soil closest to root surfaces (Marschner, 1995; Hinsinger, 1998). Chemical conditions in the rhizosphere can vary considerably from those of the bulk soil due to plant selective uptake of ions from the soil solution leading to depletion and/or accu- mulation, and release of plant exudates, such as organic anions and H+ (Marschner, 1995).
In the case of potassium, the relatively low concentration in the soil solution means that mass-flow processes (related to transpiration and plant uptake of water) are insuf-
1.2 Potassium as an essential plant nutrient ficient to meet plant potassium nutrient requirements (Marschner, 1995). This leads to a zone of depleted potassium concentration in the rhizosphere and the creation of a concentration gradient which may extend several millimetres from the root surface (Marschner, 1995). Due to the concentration gradient, plants are strongly reliant on diffusion processes for potassium nutrition, which for soils can be described using the effective diffusion coefficient, De. This is described in Equation (1.1) and is related to the diffusion coefficient in water (D1), the water content of the soil (Θ), the tortuosity factor (describing the pathway that ions follow in the soil solution and pore space) (f), and the concentration of the ion in the soil solution, C1, and the sum of ions both in the soil solution and which can be released from the solid phase (Cs) (Marschner, 1995).
De =D1Θf.C1
Cs (1.1)
Potassium uptake is also strongly related to the water content of the soil, which affects the distribution of nutrient uptake between top- and subsoil sources (Marschner, 1995).
During times of drought, contact between roots and soil solution in the topsoil is limited, and thus a significant proportion of potassium nutritional needs are met through uptake from subsoil sources. For instance, (Fox and Lipps, 1960) showed that more than 60% of total nutrient requirements were met through subsoil uptake although this accounted for only 3% of overall root mass. Timing and intensity of rainfall events during the growing season is also associated with variations in root distribution between top- and subsoils, which again affects the distribution of nutrient uptake between the two (Marschner, 1995).
A further association can be made between uptake and soil texture, which is a controlling factor for contact between plant roots and the soil matrix, however root-interception is a minor contributor to plant potassium nutrition.
Potassium cycling
Potassium contents in topsoils range from 11.7 to 26.3 mg g−1, corresponding to 40,000 to 50,000 kg ha−1 (Foth, 1991). A simplified potassium cycle is shown in Figure 1.5 where the main inputs for soil potassium are fertilisers (chemical and manure), crop residues, and soil potassium minerals (atmospheric inputs resulting from deposition of dust particles are usually insignificant), while depletion occurs primarily through crop
Figure 1.5: Simplified conceptual scheme of potassium cycling in soil (adapted from (Mengel et al., 2001; Sparks, 1987).
export, and erosion and runoff. Losses due to leaching tend to be minor, due to the potassium fixing capacity of many soils with the exception of very sandy soils (Mengel et al., 2001; Sparks, 1987).
In soils, 90-98% of this potassium is found in the crystal structure of K-feldspars and micas such as muscovite and biotite (Sparks, 2001). This potassium is released only slowly to plants through the weathering and dissolution of the mineral structure in contact with
1.3 Soils and pedogenesis the soil solution, releasing K+ in addition to other cations (Velde and Meunier, 2008).
Under natural conditions, mineral potassium is the most important source of potassium for plants, and thus soil fertility is strongly related to the soil parent material and the rate of weathering processes (Sparks, 1987). The second pool of soil potassium is so-called non-exchangeable potassium, which is fixed in the interlayer spaces of secondary 2:1 clay minerals (see Section 1.5.3), and accounts for 1-10% of soil potassium reserves (Sparks, 2003). The remaining 1-2% of soil potassium is in equilibrium between the soil solution (0.1-0.2%) and the exchangeable pool (Sparks, 2001). Potassium in the exchangeable pool consists of potassium adsorbed on mineral surfaces, such as that of kaolinite, and in the interlayer spaces of 2:1 minerals such as smectites, and associated with organic matter (Sparks, 1987). Exchangeable potassium is directly available to plants through processes
of cation exchange (Mengel et al., 2001).
1.3 Soils and pedogenesis
Soil
Difference authors have descirbed soil in various ways. For instance, Joffe (1936) described soil as
“...a natural body, differentiated into horizons of mineral and organic constituents, usually unconsolidated, of variable depth, which differs from the parent material below in morphology, physical properties and constitution, chemical properties and composition, and biological characteristics.”
while Ramann (1928) used a definition of soil as
“...the upper weathering layer of the solid earth crust.”
In contrast, Hilgard (1921) chose to focus on the interaction with plants, giving
“The more or less lose and friable material in which, by means of their roots, plants may or do find a foothold and nourishment, as well as other conditions of growth.”
Despite these of the various definitions, soil can be characterised as a complex, open physical system, meaning that material or energy may be added to or removed from the
soil (Sposito, 2008). Soils represent the intersection of, and interface between, atmosphere, hydrosphere, biosphere, and lithosphere and are made up of assemblies of various solid components such as crystalline and amorphous phases, organic matter (humus), liquids, gases and organisms (Velde and Barr´e, 2009). One of the most important defining properties of soils is their anisotropy, in that there exists a spatial distribution of chemical and physical properties (Jenny, 1994). Other typical properties include colour, pH, and texture (Soil Survey Staff, 1999). The potassium content of soils is strongly dependent on the nature and quantity of the crystalline phases which make up the pools depicted in Figure 1.5 (Sparks, 1987).
Soil forming factors
Soil mineralogy is directly related to the pedogenesis of the soil - the formation of soil, over time, from parent rock. Using the approach of the Dokuchaev School of Soil Science, the five major factors which control the genesis of soils are typically considered: parent material (p), climate (c), relief (or topography, r), biota (b) and time (t) (Figure 1.6).
These factors also indicate the openess of the soil system - climatic effects are dictated by the atmosphere and do not change dramatically when crossing the air-soil boundary.
Different combinations of these factors lead to the development of soils with specific chemical properties. They also lead to the development of soil horizons, or strata, within a soil profile which recognise vertical transitions in the degree of weathering as soils are altered downwards from the original soil-rock interface.
Climate and topography
Climatic processes such as rainfall, wind and temperature determine worldwide distribu- tions of soil. Overall, global air-flow and temperature variations tend to give high rainfall in zones close to the equator, followed by desert climates typified by high temperatures and extremely low rainfall bordering tropical zones which then graduate to temperate zones at higher latitudes (Figure 1.7) (Velde and Barr´e, 2009). The combination of high temperature and high rainfall can lead to an intense alteration, while lower rainfall and moderature temperature can lead to a significant deposition of organic matter (Jenny, 1994). This distribution of such zones of temperature and rainfall is next impacted by large-scale geological constraints such as the location and breadth of continents and mountain chains, particularly in the Northern Hemisphere (Velde and Barr´e, 2009). This shows that not only local, but global topography affects soil formation due to the effect on
1.3 Soils and pedogenesis
Figure 1.6: The genesis of soils is controlled by the five factors shown here, according to the equation of Jenny (1994).
the hydrosphere. For example, mountain chains redirect atmospheric flows of moist air in the atmosphere which then affects rainfall, while the flow of water and erosion/depostion of fine particles on a mountainside is different to that of a valley (Velde and Barr´e, 2009).
Mountains also affect local climatic factors such as temperature and water-table depth Kut´ılek and Nielsen (2015).
The flow of water is the most important agent in soil formation, as its movement allows for the precipitation, dissolution and transport of substances within the soil system. Thus rainfall has a significant role, however this is attenuated by the amount of evaporation (related to temperature), evapo-transpiration rates of overlying vegetation and the permeability of underlying soil and rock horizons.
Parent material
The composition of the parent material has an influential role in determining a) the rate of weathering and b) the chemical composition of altered minerals (Meunier, 2005).
The presence of silicon or aluminium rich parent material can lead to the formation of clay minerals, while other metal cations can influence soil colour and redox processes (Fe, Mn...), soil flocculation (Ca2+, Mg2+) or dispersion (K+, Na+), and acidic parent material again affects soil pH and dissolution rates (Jenny, 1994). These are important
Figure 1.7: Depiction of how rainfall variation with increasing distance from the equator (Velde and Barr´e, 2009).
considerations given that approximately 50-66% of soil volume consists of solid material (Sposito, 2008). In the majority of soils more than 90% of this solid material is inorganic (or mineral) (Sparks, 2003). Weathering through physical disintegration of the parent rock may results from factors such as temperature-based, stress-inducing, volume changes which cause cracking and disruption of the compact rock matrix (Kut´ılek and Nielsen, 2015). Other methods of physical weathering are freeze-thaw cycles of fluid-filled cracks, growth of plant roots, and the action of wind and glaciers (Kut´ılek and Nielsen, 2015).
Other weathering processes are due to chemical decomposition, through the movement of water through fractures and faults in the rock leading to dissolution, transport, and reprecipitation of mineral elements within the soil system (Meunier, 2005). Chemical weathering depends strongly on the nature of the chemical bonds in the parent material - i.e. on the chemical composition - as these dictate slight differences in solubility and thus which minerals are most susceptible to weathering (Kut´ılek and Nielsen, 2015). The presence of trace elements or substutions in the crystal lattice also plays a role as these can involve a distortion in the lattice and a weakening of bonds surrounding the site of substitution (Wells and Norrish, 1968).
Biota
The presence and nature not only of vegetation but also of micro-, meso- and macrofauna in the soil ecosystem is closely linked to the formation of soils. Not only is the action of plant roots important to the physical breakdown of rock structures, their exudates and secretions in the rhizosphere can accelerate chemical alteration of minerals (see
1.3 Soils and pedogenesis Section 1.8.1), and additionally all vegetal matter incorporated in the topsoil undergoes the process of humification (Sposito, 2008). A specific example of how significant the impact of biota can be occurs in coniferous forests, where under certain conditions a special type of soil called a podzols orpodosols can form . This occurs when the growth and accompanying decomposition of conifers causes acidification of the soil and leads to the leaching of all but the most insoluble elements, leaving a horizon enriched in quartz, and aluminium and iron sesquioxides (Righi and Chauvel, 1987).
Soils represent important habitats for microorganisms - one kilogram of soil can contain up to 10 trillion bacteria, 10 billion actinomycetes and one billion fungi, and large populations of organisms such as protozoa or nematodes (Sposito, 2008). Microorganisms in the soil are closely involved with processes such as the mineralisation of specific elements, in particular ‘fixation0 of nitrogen and phosphorus from organic to inorganic forms, and in the secretion of glomalin-related soil proteins (glomalin is a fungal glycoprotein believed to be associated with soil aggregration) and humification (see also Section 1.5.1) (Sposito, 2008; Kut´ılek and Nielsen, 2015). Products of microbial metabolism are associated with accelerated weathering effects on certain minerals, such as potassium bearing minerals (Song and Huang, 1988). Biotic effects also include the action of animals and man, where modern agriculture impacts soil structure (through tillage), elemental cycling (fertiliser application), and by alteration of resident plant species, while animals such as worms impact soil porosity and aggregation (Kut´ılek and Nielsen, 2015).
Effect of time
The development of a ‘well-developed0or mature soil can take centuries or millenia. Weath- ering continues progressively over time and a soil considered to be relatively young (or immature) will have only slightly altered parent material and poorly developed horizons, representing an unstable state in soil formation, while a mature soil will generally have a well-developed sequence of horizons and have reached a final state of transformation (Jenny, 1994). The estimation of age or maturity is thus based on the differentiation between horizons but this relies strongly on the assumption that all other factors (t,p,c,b) remain constant during the measured period (Jenny, 1994). There is a strong interaction between time and the factors such as climate and topography, as these affect weathering rates and the deposition or removal of material (Velde and Barr´e, 2009).
Phases of soil formation
Due to the strong interactions between the factors of soil formation one cannot discuss young or old soils in absolute terms. In certain tropical zones, the high temperature and rainfall lead to the formation of thick soils with well-differentiated horizons extremely rapidly, geologically speaking (Foth, 1991). Conversely, the high temperature and low rainfall of deserts leads to a relatively slower weathering process (Foth, 1991). Thus, it is necessary to define the stages of soil formation.
• the first phase is the weathering of the parent material by chemical dissolution and physical decomposition
• the second phase involves arrival of biotic communities, which lead to some alteration of and interaction with mineral phases by organic molecules, and to the incorporation of organic matter into the soil matrix and the presence of leaf litter
• the third phase involves the redistribution of material within the system and the differentiation between different horizons (Jenny, 1994)
1.4 Soil structure and definitions
Soils have a hierarchichal structure which exists on an increasing scale from µm (crystal- lites and microaggregates) to km (horizons and pedons). The following section attempts to describe some of the variations in soil properties as a function of scale.
Particle aggregation
Starting with the smallest scale, clay particles flocculate to form micro-aggregates or
‘quasicrystals0, which can be classed as crystallites (strongly oriented) or tactoids (irregu- larly oriented) (Meunier, 2005). Such interactions are governed by van der Waals and electrical double-layer phenomena. Organic molecules can be associated with such clay microaggregates, and are generally associated with external surfaces of the quasicrystals.
These molecules are incredibly important to maintaining the micro-aggregate structure of soils and preventing the compaction into coagulated silts and clays (Kut´ılek and Nielsen, 2015). Associations of clay microaggregates and organic molecules contribute to soil microporosity, as the organic molecules prop open inter-particulate spaces. They also act to protect the organic molecules against attack by microorganisms (Singh et al., 2017;
von L¨utzow et al., 2006). As both clay mineral surfaces and organic molecules tend to be negatively charged, these associations occur with the help of polyvalent cations such as
1.4 Soil structure and definitions Fe2+, Ca2+ etc., and also may use organic functional groups, e.g. amines (Meunier, 2005).
As the size of microaggregates increases, they may also incorporate bacteria into the structure which may confer protective benefits to bacteria against larger soil organisms (Kut´ılek and Nielsen, 2015).
Further aggregation (formation of macroaggregates) occurs on a larger scales around matter such as fine root or hyphal material and larger particles of primary minerals (i.e.
sand grains) (Huang et al., 2011). Polysaccharides are implicated strongly in cementation between larger aggregates, tending to accumulate in larger cracks and fissures where dissociation is most likely to occur (Velde and Barr´e, 2009). These largest associations, over 250µm in diameter, are influential in determining soil properties such as mechan- ical strength and aeration status, as their overall organisation determines the meso- and macroporosity of the soil (Huang et al., 2011). Macroaggregates are also the most sensitive to mechanical disruption, in particular the type that may occur under tillage agricultural systems, but also stresses such as raindrop impact (Kut´ılek and Nielsen, 2015).
Figure 1.8: Schematic representation of soil horizons, showing how development consists of downwards progression of horizontal zones with different characteristics.
Definition of soil horizons
The next level of macroscopic soil organisation is that of soil horizons. Considering that alteration takes place roughly horizontally at the interface between solid-atmosphere,
zonation then occurs where certain characteristics exist within a defined range running parallel to the interface and corresponding to an extent of alteration of the parent material (Jenny, 1994). For example, the United States Department of Agriculture (USDA) uses
the following primary designations:
• O - an organic horizon, which is dominated by detritus coming from a recognisably organic origin
• A - mineral-organic, composed primarily of mineral components which have been sufficiently altered to no longer represent the parent material, with some humified organic material present
• E - eluviated, characterised by concentrations of sand and silt particles due to the loss of clay-sized (<2 µm) particles
• B - formed below A, E, and O horizons, this is a clay rich horizon with concentrations of mineral phases conforming to significant levels of alteration or leaching
• C - represents consolidated or cemented or unconsolidated but pedogenically unal- tered parent material
• R - consolidated and continuous parent material (bedrock)
Following these general classifications to identify different horizons in a profile, more specific classification can take place using suffixes to denote certain properties.
Pedon
The pedon is a unit of study which permits to investigate the soil properties below the surface, and is the largest unit still considered a soil. It is the smallest three- dimensional sample which allows a complete description of all pedogenetic properties.
Such a description normally proceeds from a vertical profile and includes a characterisation of all soil horizons, boundary characteristics, and other elements such as colour, texture, pH, porosity, and the extent of root penetration, inter alia. The pedon also involves the precise location and description of the soil area. Pedons can also be organised on the large scale to provide a topographical indication of soil types and how they vary across polypedons across the countryside. These descriptions are based firstly on the nature of the horizons present, and are extended to take into account factors such as texture and moisture content. (Soil Survey Staff, 1999)
1.5 Soil components
Figure 1.9: Soil map of Illinois showing the variation in soil classifications througout the state.
1.5 Soil components
Humus and humification
Soil acts as a support for living beings such as nematodes, bacteria, fungi, archaea, and higher and lower plants, and these organisms all contribute to a part of the composition of the organic matter in soils (Kut´ılek and Nielsen, 2015). The most simple is the presence of dead and decaying organic matter - leaf litter, branches and root material which play host to populations of bacteria and fungi which secrete organic substances to decompose specific components such as cellulose and lignin (respectively) (Sposito, 2008).
Decomposition of the cellulose and lignin components is accompanied by the creation of new organic substances within the soil matrix. Other organisms such as ants and earthworms also play a role in the incorporation of organic matter into the soil due to physical movement of small morsels from the surface (Kut´ılek and Nielsen, 2015).
Once these larger elements of plant material are drawn beneath the surface, they are susceptible to the process of humification, which is a type of decomposition. Humification of different components of organic matter does not take place at the same rate. Sugars such as starch and glucose decompose quickly, while lignin and waxes are more resistant to decomposition, leading to the formation of soil humus consisting of a variety of organic molecules but always including high molecular weight carboxylic acids (R-COOH func- tional groups) which typify soil humus (Sparks, 2003). These acids are classified into three groups, called humins and fulvic and humic acids, based on solubility and molecular mass.
These molecules are relatively stable in soils, enduring 102-103 years in soils (Kut´ılek and Nielsen, 2015). They tend to be negatively charged due to deprotonation of the carboxyl group, leading them to attract positive cations (Hinsinger et al., 2009). The humus content of soils is strongly dependent on vegetation type and temperature, due to the dependence of humification on bacterial metabolism which is in itself suppressed by decreasing temperature, however this likely varies by exact environmental conditions.
(Sparks, 2003).
Inorganic soil constituents
The inorganic components of soils account for up to 50% of soil volume and 50-99.9%
of soil mass, and includes both primary and secondary minerals. Their size, surface, and charge impact many important soil processes and equilibria. Primary minerals are directly inherited from the parent material in that they are minerals which have not been
1.5 Soil components Table 1.2: Concentration of the ten most abundant elements and selected metals in the Earth’s crust, adapted from Wedepohl (1995).
Element ppm Element ppm
O 472000 Mn 716
Si 288000 Ba 584
Al 79600 Sr 333
Fe 43200 Zr 203
Ca 38500 Cr 125
Na 23600 V 98
Mg 22000 Rb 78
K 21400 Zn 65
Ti 4010 Ni 56
C 1990 Cu 25
chemically altered since their deposition. This is in contrast to secondary minerals which are generally formed from weathering processes acting on primary minerals (Sposito, 2008). Soil minerals tend to be ionic solids, although most bonds tend to have a degree of covalency, and their structures are formed on the basis of Pauling’s Rules governing the coordination (or arrangement) of ions based on the ratio of the ionic radii and bond-strength to minimise the charge and number of components in a crystal structure (Sparks, 2003). Silicate minerals and quartz are the most common minerals in the crust, as silicon and oxygen atoms (or ions) are the two most common elements found in the Earth’s crust (Table 1.2). The most important potassium bearing silicate minerals in soils are feldspars, micas and clays minerals and the following sections cover their structure and properties.
Structural building blocks
The basic building block of silicates is the SiO4−4 tetrahedron. This is formed from an arrangement of closely packed oxygen anions, in which four O2− anions situated at the four apices of a tetrahedron, three at the base (basal oxygens) and one anion sitting on top of the three, termed the apical oxygen. This arrangement leaves a small central cation site, which can be occupied by small cations such as silicon, Si4+ (Figure 1.10a) in four- fold tetrahedral coordination. Some substitution of the central Si4+ is possible however
Figure 1.10: Structural components of common soil silicate minerals: a) example of a silicon cation tetrahedrally coordinated to four oxygen anions, and the tetrahedral sheet formed by corner-sharing of basal oxygen; b) octahedrally coordinated cation and the octahedral sheet formed by edge-sharing of octahedra, and examples of layer-silicate structures formed by combinations of tetrahedral and octrahedral sheets c) 1:1 layer (TO), d) 2:1 layer (TOT) non-expandable, 3) expandable 2:1 layer and f) 2:1:1 layer (TOT:O).
this is limited to cations with small ionic radius such as Al3+as dictated by Pauling’s Rules.
Larger cations are found in so-called octahedral sites. These are formed from planar arrangements of three O2− anions, where each plane is rotated by 60 ◦ relative to the preceeding plane of anions. This leaves a cation site between six oxygen anions, giving six-fold octahedral coordination (Figure 1.10b). This site is commmonly populated by Al3+, but is able to accomodate cations with greater ionic radii such as Fe3+, Fe2+ and Mg2+. Ions such as K+ or Ca2+ are prevented from occupying such sites due to their extremely large ionic radii.
1.6 Minerals of interest These two substructures can then be assembled together in various ways to form different structures by sharing of the oxygen atoms. Figure 1.10 gives a basic view of some of these structures.
1.6 Minerals of interest
Feldspars
Feldspars are members of the tectosilicate family, in which SiO4−4 tetrahedra share all four oxygen atoms in a three-dimensional array. Feldspar structure is based on chains of four-membered rings of tetrahedra, while individual chains are cross-linked. The general formula can be represented by XAl(Al,Si)Si2O8, where substitution of Al3+ for Si4+ in tetrahedral sites leads to incorporation of charge-balancing cations (X) such as Ca2+, K+ or Na+ in interstitial sites within the tetrahedral network. The degree of substition and nature of interstitial cations can vary such that feldspars exists in a limited solid-solution series. These cations are surrounded by the silicate framework consisting of strong O-Si-O bonds. These bonds are resistant to weathering and as such feldspars release only slowly potassium held in interstitial positions, however attack by or- ganic acids in soils (particularly in the rhizosphere) can accelerate the rate of dissolution ().
Phyllosilicates
Micas and clay minerals belong to the group of phyllosilicates whose name derives from the Greek phyllon, meaning leaf, as a reference to their layered structures. All members of the phyllosilicate family share the same two sheet structures, and different minerals are formed from different combinations of the two sheets. The two types of sheets are as follows:
• tetrahedral sheet: SiO4−4 tetrahedra which share all three basal oxygen with adjacent tetrahedra, while protons are associated with apical oxygen atoms for reasons of charge balance. Different degrees of substition of Al3+ (or rarely Fe3+) for Si4+
exist. Tetrahedral sheets are not found as independent minerals but are always associated with octahedral sheets
• octahedral sheet: two oxygen atoms are shared between adjacent AlO−96 octahedra in an edge-sharing arrangement. Protons are also associated with O2− for charge balance. Al3+ can be substituted by a variety of atoms, and the sheets can be di-
or tri-octahedral, where in dioctahedral minerals only two of every three octahedra are occupied by M3+. In trioctahedral minerals all three octahedra are occupied by M2+. Octahedral sheets do exist as independent minerals in the form of metal hydroxides, but these are not silicates although they are structurally analogous to octahedral sheets.
Bonding between tetrahedral and octahdral sheets occurs through the sharing of tetrahedral apical O2− ions and those at the corner of octahedra. This is accompanied by some distortion of the tetrahedral sheet to account for misfit between the dimensions of the two sheets. Substitution of tetrahedral or octahedral cations (Si4+ or Al3+) may lead to charge deficiency termed the ‘layer charge ’. As a result of layer charge, interlayer cations are present to balance the charge and these are generally situated in the hexagonal cavities created by the rings of six tetrahedra in the tetrahedral sheet. Families of minerals can be characterised by the association of either one tetrahedral and one octahedral sheet (1:1 or TO) or by two tetrahedral and one octahedral sheets (2:1 or TOT sandwich), and further as TOT:O (2:1:1) where an additional octahedral sheet is found between TOT layers (Figure 1.10).
Micas and 2:1 clay minerals
At this point it is necessary to differentiate between micas and the clay minerals found in soils. This separation is done based on the definition of clay minerals by Guggenheim and Martin (1995) as
“naturally occurring material composed primarily of fine-grained minerals, which is gener- ally plastic at appropriate water contents and will harden when dried or fired. Although clay usually contains phyllosilicates, it may contain other materials that impart plasticity and harden when dried or fired. Associated phases in clay may include materials that do not impart plasticity and organic matter.”
where their size is limited to particles with a diameter of less than two microns. This size limit is somewhat arbitrary, arising from the minimum particle size that was able to be investigated by classical optical and single-crystal microscopic methods before the invention of electron microscopy. However, despite their structural similarity, particles of muscovite and biotite are rarely found in the <2 µm fractions but rather in the silt (50-2 µm) and sand (2000-50µm) fractions of soils. Micas tend to be primary minerals inherited from the parent material as they are commonly found in shales, granites and
1.6 Minerals of interest Table 1.3: Clay mineral groups as defined by differences in layer charge and interlayer distance adapted from data in Brindley and Brown (1980). Where two names or structural formulae are given, the trioctahedral endmember is specified first followed by the dioctahedral end-member.
Layer-to-layer distances are provided in hydrated conditions.
Family Layer
type
Structural formulae Layer charge, meq 100g−1
Layer-to- layer distance, ˚A Serpentine
—kaolin
TO Mg3Si2O5(OH)4
Al2Si2O5(OH)4
∼0 7.1-7.3
Talc
—pyrophyllite
TOT Mg3Si4O10(OH)2 Al2Si4O10 (OH)2
∼0 9.1-9.4
Micas TOT (Al,Fe)2(Al,Si)4O10 (OH)2 .(K,Na,Ca)
∼1 or ∼2 9.6-10.1 Smectites TOT (Al,Mg,Fe)2(Al,Si)4O10
(OH)2 .(K,Na,Ca)nH2O
0.25-0.6 14.4-15.6 Vermiculites TOT Mg3(Si3Al)O10(OH)2
.(M+,M2+)nH2O
0.6-0.9 14.4-15.6 Chlorites TOT:O (M2+3−y−z,M3+y , z)
(Si4−x,M3+x )O10 (OH)2 .(M2+,M3+)3(OH)6
variable ∼14.1
schists, inter alia. During the weathering process they are thought to lose some of their interlayer potassium (among other slight compositional changes) to form illites, secondary minerals which are precursors to smectites and vermiculites (Moore and Reynolds, 1997).
Smectites, vermiculites, illites and micas are members of a sequence of identical TOT structures, with one difference being increasing layer-charge through the series (Moore and Reynolds, 1997). These minerals generally tend to form interstratified crystallites where smectite layers may be found between layers of illitic nature, or vice versa. Chlorites are also commonly found interstratified with 2:1 layers (Brindley and Brown, 1980).
Layer stacking and disorder
These layered structures lead to some shared characteristics, such as a tendency to cleave along the horizontal plane between layers as a result of the weaker interlayer bonds.
They also have similar platy particle morphology, which result from the stacking of many clay layers and where the number of layers determines the size or thickness of the crystal. However, the stacking of the layers can give rise to different degrees of
disorder within the crystal. As there is little energetic difference between different layer stacking configurations, and the interaction between adjacent layers is weak, layers can be translated or rotated with respect to one another (Drits and Tchoubar, 1990). If the TO or TOT layers are randomly stacked with no relation between the translational or rotational position of sequential layers, they give rise to a structure termed ‘turbostratic’.
This is common in smectites as there is little association between layers to determine their position relative to one another. If the translation and/or rotation between consecutive layers is both regular and fixed, and the magnitude of which corresponds to the symmetry of the silicate layers, then this gives rise to ordered, repeating stacked structures which are termed polytypes (Brindley and Brown, 1980).
1.7 Characteristics of 2:1 minerals
Layer charge
As previously described, cation substitutions within tetrahedral and octahedral sheet can lead to charge deficit and overall negative charge on the sheet. This type of charge is termed ‘permanent charge’, and is generally compensated for by interlayer cations.
Selectivity for charge balancing cations may be displayed, roughly according to the following series:
1+: Li >Na > K> NH4 >Cs 2+: Mg>Ca >Sr >Ba
Vermiculites are an exception to this series as they tend to prefer Mg2+ over Ca2+
(Meunier, 2005). This cation selectivity occurs based on the differences of size, hydration energy, and the amount and location of layer charge. Layers also have another type of charge described as variable charge, which arises from unfufilled -O or -OH bonds at the crystal surface. These bonds can be satisfied with additional OH− or H+ ions, meaning this charge can change depending on protonation or deprotonation as a result of local pH conditions, and it also controls dispersion and flocculation behaviour due to the attraction or repulsion between particles as a result of surface charge. Variable charge is most significant for minerals with little to no permanent charge such as kaolinite or talc, and loses importance as permanent charge increases (Brindley and Brown, 1980).
